The present invention relates generally to a photolithographic molecular fluorine laser system, and more particularly to a two-stage laser mode of photolithographic molecular fluorine laser system in which the center wavelength of an oscillation-stage laser is matched to the center wavelength of an amplification-stage laser.
For photolithographic technologies to achieve on semiconductors a semiconductor integrated circuit having a marking width of 70 nm or less, there are demanded exposure light sources of wavelengths of 160 nm or less. F2 (molecular fluorine) laser systems that give out ultraviolet rays of wavelengths of around 157 nm are now thought of as a promising candidate for those light sources.
As shown in FIG. 16, a typical F2 laser system has two primary oscillation wavelengths (xcex1=157.6299 nm and xcex2=157.5233 nm: Sov. J. Quantum Eelectron. 16(5), May 1986), with its spectral linewidth (FWHM: full width at half maximum) being of the order of about 1 pm. The intensity ratio of two such oscillation lines is I(xcex1)/I(xcex2)≈78. For exposure purposes, usually, the oscillation line of wavelength xcex1(=157.6299 nm) having stronger intensity is used.
By the way, the semiconductor photolithography technology is roughly broken down into the following two types that use:
1) a dioptric system, and
2) a catadioptric system.
The use of the catadioptric system for photolithography ensures that chromatic aberrations are reduced. For this reason, an aligner using such a catadioptric system holds great promising in a wavelength range of the order of current 157 nm. However, the catadioptric systems are more troublesome than conventional dioptric systems in terms of the optical axis alignment of an aligner.
On the other hand, the dioptric system is a projection optical system generally used on heretofore known semiconductor aligners. One grave problem with the semiconductor photolithographiy technology is how to correct an optical system for chromatic aberrations. In the dioptric system, correction of chromatic aberrations has been achieved by some combinations of optical elements such as lenses having varying refractive indices Because of considerable restrictions on the types of possible optical materials that are transparent to a wavelength range in the vicinity of 157 nm, however, there is still no option but to use CaF2.
Thus, some line-narrowing means is needed for F2 laser systems used as a light source for the dioptric type aligners. For instance, the spectral full width at half maximum of a laser beam should be narrowed to 0.3 pm or less.
On the other hand, the average output necessary for an F2 laser used as a photolithographic light source, for instance, is 20 W. To put it another way, when the repetitive frequency of the F2 laser is 2 kHz, a pulse energy per pulse is 10 mJ, and at the repetitive frequency of 4 kHz, a pulse energy per pulse is 5 mJ.
However, when one wishes to obtain laser outputs of 5 to 10 mJ while, for instance, a coated etalon is located as line-narrowing means in a laser resonator, the coating of the etalon is damaged; there is no option but to use an etalon without any coating thereon. Accordingly, it is still impossible to make spectral linewidths narrow. In addition, only linewidths of 0.4 to 0.6 pm are merely obtained at best because of the presence of much ASE (amplified spontaneous emission) component. Thus, it is still difficult to achieve any narrow band at pulse energies of 5 to 10 mJ.
Situations being like this, for instance, it is reasonalbe to rely on a two-stage laser system comprising an oscillation-stage laser and an amplification-stage laser for the purpose of obtaining laser beams having spectral linewidths of 0.3 pm or less and pulse energies of 5 mJ or greater. To be specific, the oscillation-stage laser gives out a laser beam that has low outputs yet spectral linewidths of 0.3 pm or less. Then, this laser beam is amplified at the amplification-stage laser to obtain a laser beam that has spectral linewidths of 0.3 pm or less and pulse energies of 5 mJ or greater.
A typical two-stage laser system operates in two modes, the injection locking mode and the master oscillator power amplifier (MOPA) mode. In the injection locking mode, the amplification-stage laser is provided with a laser resonator for which an unstable resonator is used. In the MOPA mode, no laser resonator is used. In the MOPA mode where no laser resonator is provided to the amplification-stage laser, the amplification-stage laser functions as a one-pass amplifier for a laser beam coming from the oscillation-stage laser.
For a line-narrowing element for the oscillation-stage laser, some combinations of one or more combined beam expansion and dispersion prism groups with gratings, a combination of an etalon with a total reflecting mirror, or the like are used. In what follows,-exemplary constructions of two-stage laser systems operating in the injection lock and the MOPA modes are explained.
FIG. 17(a) is illustrative of the injection locking mode where a prism and a diffraction grating are used as line-narrowing element and FIG. 17(b) again of the injection locking mode where an etalon is used as a line-narrowing element. FIG. 18(a) is illustrative of the MOPA mode where a prism and a diffraction grating are used as line-narrowing element and FIG. 18(b) again of the MOPA mode where an etalon is used as a line-narrowing element. Throughout FIGS. 17(a) to 18(b), reference numeral 10 represents an oscillation-stage laser, 20 an amplification-stage laser, 1 a laser chamber in the oscillation-stage laser 10, 1xe2x80x2 a laser chamber in the amplification-stage laser 20, 2 a line-narrowing module (line-narrowing element), 3 an output mirror in the oscillation-stage laser 10, 4 an aperture for limiting a laser beam in the oscillation-stage laser 10, 5 a diffraction grating that forms a part of the line-narrowing module 2, 6 a combined beam expansion and dispersion prism that forms a part of the line-narrowing module 2, 7 a concave mirror that forms a part of an unstable resonator in the amplification-stage laser 20, 8 a convex mirror that forms a part of the unstable resonator in the amplification-stage laser 20, 9 a reflecting mirror interposed between the oscillation-stage laser 10 and an amplification-stage laser 20, 11 an etalon that forms a part of the line-narrowing module 2, and 12 a total reflecting mirror that forms a part of the line-narrowing module 2.
Specific reference is here made to the injection locking mode of FIG. 17(a), wherein the prism 6 and diffraction grating 5 are used as the line-narrowing element 2. The oscillation-stage laser 10 has a function of giving out a seed laser (seed laser light) for the laser system, and the amplification-stage laser 20 has a function of amplifying that seed laser. Namely, the overall spectral characteristics of the laser system are determined by the spectral characteristics of the oscillation-stage laser 10. Then, laser outputs (energy or power) from the laser system are determined by the amplification-stage laser 20. The oscillation-stage laser 10 comprises the line-narrowing module 2 including the expanding prism 6 and diffraction grating 5, so that laser beams having narrowed spectra linewidths are produced from the oscillation-stage laser 10.
It is here noted that the line-narrowing module 2 may be made up of the etalon 11 and total reflecting mirror 12, as shown in FIGS. 17(b) and 18(b).
The laser beam (seed laser beam) from the oscillation-stage laser 10 is guided to and poured into the amplification-stage laser 20 via a laser propagating system including the reflecting mirror 9 or the like. In the injection locking mode (FIG. 17), the amplification-stage laser is built up of the concave mirror 7 and convex mirror 8 so that the laser can be amplified even at a small input. For instance, an unstable resonator having a magnification of 3 or greater is used to this end.
There is a hole in the concave mirror 7 that forms a part of the unstable resonator in the amplification-stage laser 10, so that the seed laser beam introduced through that hole is reflected and expanded at the convex mirror 8 as shown by an arrow. Then, the reflected and expanded laser beam passes effectively through a discharging section in the laser chamber 1xe2x80x2, so that the power of the laser beam increases. Then, the laser emerges from the convex mirror 8. The concave mirror 7 is provided in its center with a spatial hole and on the rest with a mirror coating having high reflectivity. The convex mirror 8 is provided at its center with a high-reflectivity mirror coating and with an antireflection coating at a portion around that center, from which the laser beam is to emerge. For the concave mirror 7, it is acceptable to use a mirror substrate having a spatial hole which alone is provided with an antireflection coating. It is also acceptable to use an unstable resonator comprising a mirror free from a transmitting portion.
A molecular fluorine laser uses helium and/or neon gases as buffer gases for both the oscillation-stage laser 10 and the amplification-stage laser 20. Whenever necessary, xenon may be used in combination.
FIG. 19 is illustrative in schematic of the laser pulse waveform vs. change-with-time of spectral linewidth relationships in a two-stage laser mode of F2 laser system. In FIG. 19, t as abscissa is indicative of time, I as ordinate of intensity, and xcex94xcex of a spectral linewidth. As can be seen from the pulse waveform of FIG. 19, the spectral linewidth becomes slender with the lapse of time (i.e., as the number of round trips in the resonator increases). In a conventional band-narrowing laser, the spectral linewidth takes the form of an integral value. In the two-stage laser mode, however, it is possible to extract the latter part of a pulse, whose linewidth becomes instantaneously narrow, and amplifying only the output while that linewidth is kept, thereby reducing the ASE component.
By use of such a two-stage laser mode, it is thus possible to achieve high outputs simultaneously with very narrow bands.
When F2 laser systems are used as photolithographic light sources, the spectral purity is critical. The term xe2x80x9cspectral purityxe2x80x9d is one index to the degree of concentration of spectral energy, referring to a linewidth that includes a xe2x80x9ccertain area ratioxe2x80x9d in a spectral waveform. For instance, the xe2x80x9c95% purityxe2x80x9d commonly used in the art refers to a linewidth that accounts for a 95% area of the total area of a spectral waveform, as measured from the center side as shown in FIG. 20.
Referring specifically to an F2 laser system, the center wavelength differs between upon buffered with helium and upon buffered with neon; for instance, when an oscillation-stage laser using a neon buffer and an amplification-stage laser using a helium buffer are immediately oscillated, they will be synchronized with the center wavelength of the oscillation-stage laser in misalignment with that of the amplification-stage laser, as shown in FIG. 21(a). Because, by definition, the F2 laser is narrower in linewidth than ArF or KrF excimer lasers, this center wavelength misalignment will lead directly to deterioration in the spectral purity of a laser beam after amplification, as shown in FIG. 21(b).
In view of such situations as mentioned above, an primary object of the invention is to provide a photolithographic molecular fluorine laser system operating in a two-stage laser mode, wherein the center wavelength of an oscillation-stage laser is matched to that of an amplification-stage laser, thereby oscillating a laser beam having a low spectral purity and a narrow linewidth.
According to the present invention, this object is achievable by the provision of a photolithographic molecular fluorine laser system that comprises an oscillation-stage laser and an amplification-stage laser and operates in a two-stage mode, wherein:
the center wavelength of a laser beam emitted out of the oscillation-stage laser is compared with and substantially matched to the center wavelength of a laser beam emitted out of the amplification-stage laser when said amplification-stage laser is oscillated by itself.
According to the invention, the center wavelength of a laser beam emitted out of the oscillation-stage laser is compared with and substantially matched to that of a laser beam emitted out of the amplification-stage laser when the amplification-stage laser is oscillated by itself. It is thus possible to make the band of the photolithographic molecular fluorine laser system very narrow while the spectral purity is satisfactory.
Referring here to the xe2x80x9ccenter wavelengthxe2x80x9d of the laser beam emitted out of the amplification-stage laser when oscillated independently, this term means, in the case of the injection locking mode, literally the center wavelength of the laser beam emitted out the amplification-stage laser when oscillated independently and, in the case of the MOPA mode, the wavelength that is found in the vicinity of any one of primary wavelengths emitted out of the amplification-stage laser and has the highest gain (see FIG. 16).
Still other objects and advantages of the invention will be in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which sill be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.